(1) Field of the Invention
The invention relates to a method and device for electrolytic treatment of electrically conductive surfaces of mutually isolated sheet and foil material pieces and applications of the method, especially for producing circuit boards and conductive foils.
(2) Description of the Related Art
Electroplating processes are used to produce circuit boards and conductive foils in order either to deposit metal or to implement other electrolytic treatments, for example metal etching methods. For quite a number of years, so-called continnous system have been used for this purpose, the material being transported through said systems in a horizontal direction and, during transportation, being brought in contact with treatment fluid.
Such a continnous system is described for example in DE 36 234 481 A1. This unit has anodes, current supplies to the circuit boards to be coated and conveying means. The conveying means are configured as a continuous, revolving actuated row of individual clamps which hold the lateral edges of the circuit boards securely and move in the conveying direction. Current is supplied to the circuit boards via these clamps. For this purpose, the clamps are provided with current via brush arrangements.
Another type of electric contact and way of conveying circuit boards in a continnous system is described in DE 32 36 545 C3. In this case, contact wheels are used instead of clamps, said wheels rolling on the moved circuit boards and providing electrical contact with the circuit boards in this way.
Both systems must be elaborately constructed in order to be able to transfer the at times large metallising currents to the circuit boards. In the case of very high metallising currents, there are still no satisfactory solutions since fundamentally contact resistances occur at the contacts (clamps, contact wheels) so that the contact points can be heated to a very high degree from time to time by the current flow and the contacted metal surface can be damaged. This disadvantage is revealed in particular in those materials to be treated, which have, as in the case of circuit boards and conductive foils, a very thin conductive layer, usually of copper, on an insulating core layer. This thin layer can easily “burn through” when sufficiently large currents are used. The device of DE 36 32 545 C3 has the further disadvantage that metal is also deposited on the contact wheels, and the metal layer, especially on the bearing surfaces, can present problems. Only by dismantling the wheels and subsequently removing the deposited metal layer can this problem be resolved.
A fundamental disadvantage of this device resides in the fact that only whole-surface conductive surfaces can be electrolytically treated but not electrically mutually insulated structures.
As a solution to the latter problem, a method has been proposed in WO 97/37062 A1 for electrochemical treatment of electrically mutually insulated regions on circuit boards. Accordingly, the circuit boards, which are brought in contact with the treatment solution, are brought in contact successively with stationary brush electrodes, which are supplied from a current source, so that an electrical potential can be applied to the individual electrically conductive structures. An electrical potential is applied between the brushes, which are preferably formed of metal wires, and the anodes, which are disposed between the brushes.
This device has the disadvantage that the brushes are completely covered with metal within a very short time since approximately 90% of the metal is deposited on the brushes and only 10% on the regions to be metallised.
Therefore, the brushes must be freed again of metal after just a short operational time. For this purpose, the brushes must be dismantled again from the device and be freed of metal or else elaborately constructed devices need to be provided which help to remove again the metal on the brushes by means of electrochemical polarity reversal of the brushes to be regenerated. In addition, the brush ends can easily damage fine structures on the circuit boards. Likewise, the brush material thereby wears quickly, the finest particles being rubbed off and getting into the bath where they lead to damage during metallisation. Especially for metallising very small structures, for example those with a width or length of 0.1 mm, there must be used brushes with very thin wires. These wear especially quickly. Particles which come from the worn brushes then proceed into the bath and into the holes of the circuit boards and produce significant defects.
In other known methods for metallising electrically insulated structures on circuit board material, currentless metallising processes are used. However, these methods are slow, difficult to implement and expensive since fairly large quantities of chemical substances are used. The used substances are frequently environmentally damaging. They therefore incur further significant costs in disposing thereof. In addition, it is not ensured that only the electrically conductive structures are metallised. It is often observed that, in this case, the metal is also deposited on the electrically insulating surface regions which lie between, resulting in rejection.
Methods are known for electrolytic etching, pickling and metallising of metal strips and metal wires which methods are effected without electrical contact of the strips and wires:
A method is described in EP 0 093 681 B1 for continuous coating of wires, tubes and other semi-finished products made of aluminium with nickel. In this method, the semi-finished product is firstly conveyed into a first bath container and then into a second bath container. In the first bath container, the semi-finished product is guided past a negatively polarised electrode and, in the second bath container, guided past a positively polarised electrode. A metallising bath is situated in the bath containers. As a consequence of the fact that the semi-finished product is electrically conductive and, at the same time, is in contact with both metallising baths, the circuit between the electrodes, which are connected by a current source, is completed. In contrast to the negatively polarised electrode in the first bath container, the semi-finished product is anodically polarised. In contrast to the positively polarised electrode in the second bath container, the semi-finished product is on the other hand cathodically polarised so that metal can be deposited there.
A method is described in EP 0 838 542 A1 for electrolytic pickling of metallic strips, especially special steel strips, strips made of titanium, aluminium or nickel, the electrical current being directed through the bath without electrically conducting contact between the strip and the electrodes. The electrodes are disposed opposite the strip and polarised cathodically or anodically.
A method is known from EP 0 395 542 A1 for continuous coating of a substrate, which is made of graphite, aluminium or its alloys, with a metal, the substrate being guided successively through two containers, which are connected to each other and contain an activation bath or a metallising bath, a cathode being disposed in the first container and an anode in the second container. Using this method, rods, tubes, wires, strips and other semi-finished products can be coated as substrates.
Finally, a device is disclosed in Patent Abstracts of Japan C-315, Nov. 20, 1985, Vol. 9, No. 293, JP 60-135600 A for electrolytic treatment of a steel strip. The strip is guided through an electrolytic bath for this purpose between oppositely polarised electrodes. In order to prevent an electrical current flow between the oppositely situated and oppositely polarised electrodes, shielding plates are provided between the electrodes in the plane in which the bath is guided.
The problem underlying the present invention is therefore to avoid the disadvantages of the known electrolytic treatment methods and in particular to find a device and method with which a continuous electrolytic treatment of electrically conductive surfaces of mutually isolated sheet and foil material pieces is possible at low cost, especially for producing circuit boards and conductive foils, it also requiring to be ensured that the equipment costs are low and that the method can be implemented with adequate efficiency. In particular, the method and the device should be suitable for treating electrolytically electrically insulated metallic structures.